In a world increasingly driven by content that’s hiding in an email or behind a Tweet, subject lines are more important than ever.

In advance of a talk later this week at the Internet Marketing Conference in New York, here are some of the approaches and best practices in crafting better subject lines.

#1. People open emails for three reasons. Specific self-interest, general self-interest (also known as brand) and curiosity.

The reason we’re talking about self-interest instead of “relevance” is because the term “self-interest” keeps us on point – thinking about what they want. “Relevance” should mean the same thing, but often translates into “how do I make my agenda relevant to subscribers?”

Knowing what they want comes from a collection of data points, from clicks and comments, searches and social media. The beauty of digital interaction is that our customers have an array of ways in which to tell us what they want by their explicit requests and implicit behaviors – it’s up to us to listen.

It also comes from aligning your subscribers into segments that relate to who they and not simply how they relate to your KPIs. A segment called “shoe lovers” aligns with their personality and tendencies in a way that “recent purchasers” doesn’t.

Once your database is effectively segmented, automation is your friend. Building programs that do one thing well can be very powerful, and it’s easy to write effective subject lines that can be used repeatedly. For example, once you’ve got your “shoe lovers” segment created, it’s straightforward to create a monthly email that pulls in new shoes and related offers, with a subject line that suits…”New Shoe Digest”…”July’s best new shoes”…etc.

The “From” line is by some measures the most important driver of opens. That’s a measure of the brand relationship, but it still reflects self-interest. Subscribers have some to associate value with your brand and your emails.

We tend to think of email as a direct response mechanism and measure our success in terms of opens, clicks and conversions, but there’s potential brand value in all those emails they don’t open. For most customers and many prospects, company emails are the most regular brand exposure they get. That means “From” and subject lines should be reinforcing our brand at every chance. Here are a couple of different approaches;

Take a look at a months’ worth of emails from clothing retailer Banana Republic, and you’ll see two consistent themes. The first is “Free Shipping” which appears in every single subject line. The second, appearing in most messages, is some variation on “time is running out.” These messages are not particularly “on-brand” but they are specific, relevant and probably motivating. The unopened emails definitely convey the ‘free shipping’ message even to inactive list members.

Contrast that with another clothier, Urban Outfitters. Their subject lines are typically short, and the closest to a theme is the recurrence of the word “new.” The voice is slightly edgy, like the line “Warning: Graphic,” which is on-brand for Urban Outfitters. But topically, they’re all over the place, from sales to new arrivals to contests. This might not leave the subscriber with a specific message, but it does reinforce their brand, and also allows for greater variation in response.

Consistent messaging like Banana Republic’s should be the result of long testing against significant variation like you see in the Urban Outfitters subject lines. There will be home runs and strike outs, each one informing future subject lines.

The last reason to open an email is old fashioned curiosity. You can get a huge open rate if you want it. “See Jennifer Aniston’s Bare…” is likely to generate a high click through from at least some segments. But, when they open the email to discover that it’s Jen’s Bare Naked Ladies’ songlist that you’re referring to, good will and interest will wane. There may be no such thing as bad publicity, but there is such a thing as bad open; one that harms the brand relationship instead of pushing it forward. They result from stretching the truth, playing the reader for a sucker, wasting their time and not delivering on promises.

#2. Think big first.

When it comes to testing subject lines, think about the big picture before diving into the minutiae of whether “past issues” or “recent issues” scores better (btw, it’s the second one). One way to do this is by thinking of subject lines in terms of categories – types of subject line that fit a general theme.

These will vary by mailer, but would often include direct (yes, even sales-y), educational, brand aligned and clever. The first two types are self-explanatory. Brand aligned would be using words, offers and phrases that reinforce your brand proposition; thinking not just about opens and clicks but also about the effect of those subject lines in the inbox. Clever subject lines are often losers, but it depends on your audience. If you’ve got a strong relationship with your recipients, there’s more latitude to be creative.

One of the essential issues in all of digital marketing is that it’s a lot easier to get data from surface metrics like open rate than it is to track all the way through to the important stuff – purchase, registration, etc. For this reason, marketers will optimize against open rates only to find that high opens don’t equal high dollars. Using the CTR to open ratio (divide the former by the latter) will give you an idea of relevance that’s more accurate than either one alone.

#3. Then think small.

Once you’ve thought about your email and subject line strategy, it’s worth spending time with specifics. Many studies have shown that small difference can have a large effect on open rates. For example, when referring to quantitative elements (length of sale, discounts, etc.), test using the numbers versus words. “20% Off” might work well for a higher priced category than “$20.00 Off”, while the reverse could be true at a lower price point.

#4. Email recipients are special. Let them know.

It’s helpful to think about your subscribers as being different from other customers and prospects. You’ve got access to their inbox, where they spend time every day. If your email program gives them something of value in exchange for that access (and it should), then your subject lines should remind them of that. Sometimes that’s a straight monetary benefit (extra 5% off for subscribers), but it can also be early access to products, special access to content or an email only contest or promotion.

#5. Test the important things.

In an ideal world, every subject line gets tested. In most marketing departments, that’s unrealistic, but there should always be bandwidth to test the most important subject lines;

Standard, repeating messaging – does every email begin with the name of the newsletter or your company name? Those repeating elements take up space at the beginning of your subject line, so it’s imperative that they enhance response. Are there ways to shorten the standard messaging or give it greater impact?

Welcome messages – most welcome messages waste the opportunity and do nothing more than acknowledge the new subscriber. The message and its subject line should be the first step in creating an email relationship that delights and serves the customer. Is there a special benefit to being a subscriber? If so, the welcome message is a great place to reinforce it, and the subject line can reflect this.

Re-up messages – if there’s a subscription component to your business, the messages around getting a re-up should be tested mercilessly. Making them about the customer and their use of your service is one direction that can work. For light users, emphasizing the untapped potential of the service is important, while heavy users will respond to straightforward messages and promotions as the clock clicks down.

Service messages – many of our emails are automated reminders of things like account status, feature changes and updates. Any triggered message that goes out to large portions of your list is an opportunity to move the conversation to benefits and actions. Consider the airmiles reminders you no doubt receive every month. The subject line is usually a bland “View your current summary” or the like. Tying the content with the subject line could yield something more motivational, such as “You can go to Europe for free…who are you taking with you?”

#6. Don’t obsess over hidden words.

Every discussion of subject lines will always include a question about ‘forbidden’ words. For most mailers, the more important question is how they treat their email list and their customers in general. The principle email services are more interested in whether subscribers find your messaging relevant than whether you use “Free” in the subject line. That’s not to say that a spammy sounding subject won’t get you blocked, but that’s an occasional issue, whereas your philosophy toward your subscribers is fundamental to long term success.

#7. We can’t test until we’ve created.

Creativity and inspiration are the heart of every good piece of marketing, even in an increasingly data-driven discipline. Subject lines are no exception. In the same way that your content should be motivated by your customers, subject lines should flow from the content. If the piece is educational, what did you learn? If it sells products, what’s the most appealing one? Why? Tie into the outcomes that matter to your subscribers and they’ll be eager to open your emails.